Considerable commercial success has now been attained with metallocene catalysts; both supported and homogeneous. Success has been in product design for LLDPE, VLDPE, plastomers, and elastomers, where control of linear alpha-olefin comonomer incorporation efficiency and composition distribution coupled with molecular weight distribution has lead to substantially enhanced properties of value to customers. These include puncture resistance, gloss/haze, tear strength, and permeability. More recently, improved melt state properties have also been brought to commercial operations with metallocene offerings such as Enable™ LLDPE.
Metallocene and other single site catalysts have had less impact in high density polyethylene where comonomer concentration is very low and consequently comonomer distribution and incorporation efficiency are less important. Nonetheless, occasionally efforts have been made to employ metallocene catalysts for HDPE design (see Macromolecular Materials and Engineering, 2005, 290(6), pp. 610-620, U.S. Pat. Nos. 7,576,163, and 7,148,298). A common strategy has involved mixing two metallocene catalysts on the same support (see Macromolecular Rapid Communications, 2001. 22(8): p. 573-578). Apparently, it has proven difficult to find two metallocenes with sufficiently different molecular weight capability to achieve the proper balance of melt rheological and solid state characteristics.
U.S. Pat. No. 7,625,982 discloses multimodal polyethylene compositions, having at least two PE components made using two catalyst compounds and specifically discloses higher density polyethylenes at Table 3, runs 10-16. Note the examples all have Mw/Mns over 4.5.
EP 659 773 B1 discloses gas phase polymerization processes at a temperature above 65° C. using unsupported catalysts for producing polyethylene containing long chain branches (up to 3 long chain branches per 1000 C's). Note the highest density in the examples is 0.940 for an MI less than 5 dg/min.
U.S. Application Publication No. 2011/0059278 discloses in Table 1, page 2, a polyethylene (R11) having a density of 0.960, a Tm of 137° C., an MI of 7.39 dg/min, and a long chain branch index of 0.14 prepared using a bridged bis(tetrahydroindenyl) based catalyst system.
U.S. Application Publication No. 2005/0020438 discloses in Table 1, page 4, Example 3, use of a supported dimethylsilylbis(tetrahydroindenyl)zirconium dichloride and MAO in a slurry polymerization to produce polyethylene having an Mw of 329,000 and an HLMI of 0.27 g/10 min.
Other references of interest include: EP 1 217 013 A2; EP 743 324 B1; and EP 700 937 B1.
Up until the time of the present invention, little progress has been made in obtaining long chain branching in HDPE products made with supported metallocene catalysts. Up to now, only supported chromium catalysts are used commercially to produce HDPEs containing a modest—but important—population of long chain branching. The broad—often bimodal—molecular weight distribution and the long chain branching offer melt strength improvements in film blowing and blow molding, respectively.
Therefore, a need exists for a product and process that overcomes one or more of the current disadvantages noted above.